Recently, for use in mobile communications, portable telephones and the like, a single antenna has come to be used dually for both signal transmission and signal reception.
More specifically, in the case that signals having different frequencies are used for signal transmission and signal reception, a duplexer is connected to the antenna, in which a bandpass filter that passes the transmission signals is used in combination with a bandpass filter that passes the reception signals, whereby the antenna can be utilized in a shared transmitting/receiving manner.
Further, in the case that the transmission signals and the reception signals both use the same frequency, a switching circuit is used, in which by switching between the transmission and reception signals on a timewise axis, the antenna also can be utilized in a shared transmitting/receiving manner.
Additionally, in such a signal transmission/reception apparatus, in particular for improving signal transmission characteristics, a low pass filter may be connected to the transmission side, for example, of a signal transmission/reception switching device.
Incidentally, a device capable of miniaturization, in which ceramic dielectric substrates are used, has been generally adopted as a low pass filter, which is connected to the transmission side of the signal transmission/reception apparatus.
Heretofore, as a low pass filter that utilizes ceramic dielectric substrates, a low pass filter suitable for use in a high frequency band of roughly 1 to 3 GHz has been proposed, having attenuation poles at two-times and three-times the passband, in which the attenuation poles are capable of being adjusted by many parameters, yet in addition, wherein the overall shape of the device can be made smaller in size. (See, for example, Patent Document 1.)
Further, a low pass filter has been proposed which exhibits no vertical directionality, with a structure such that, even if the filter is mounted with the vertical orientation thereof switched, electrical characteristics thereof are not changed. (See, for example, Patent Document 2.)
Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-153414.
Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-110393.